FIG. 9 shows a schematic view of a conventional suction cup. The conventional suction cup includes a suction disc 90, a shell 91, and a flip unit 92. The shell 91 is further disposed with a connection element 911 for connecting to a heavy object 95 (the connection element and the heavy object are depicted as a block in the figure). The heavy object 95 can be any object, which is hanged by the suction cup to a vertical contact surface 96. The suction cup employs the suction disc 90 to attach to the contact surface, and a variable space 901 is formed between the suction disc 90 and the contact surface 96. The flip unit 92 draws the center area of the suction disc 90 away from the contact surface 96 so that the variable space 901 is expanded to generate a suction force similar to the vacuum suction. In addition, the flip unit 92 applies a pressure during the flipping to the shell 91 so that the shell 91 presses tightly against the suction disc 90 to prevent outside air from entering the variable space 901. In other words, the longer the shell 91 presses tightly against the suction disc 90, the longer the suction attachment will last.
However, the suction cup does not always behave in an idealistic manner. The following description refers to the directions shown in FIG. 9. In general, a suction cup is attached in a vertical state to the contact surface 96. But when the heavy object 95 is hanged on the connection element 911 of the shell 91, the lower contact edge 912 of the shell 91 applies the most force to the suction disc 90. In contrast, the upper contact 913 of the shell 91 does not contact the suction disc 90 at all. The distribution of the applied force affects the attachment of the suction disc 90. Using a horizontal line crossing the center of the suction disc 90 as a division, the contact area between the suction disc 90 and the contact surface 96 above the line is smaller than the contact area below the line so that the shape of the cross-section of the variable space 901 is distorted to become a water drop. As a result, the drawing of the bottom half will continuously pull the upper half of the suction disc 90 away from the contact surface 96. With the upper contact edge 913 pressing against the suction disc 90, the outside air will seep into the variable space 901 from the upper half of the suction disc 90 until the suction disc 90 peels off from the contact surface 96. It is desirable to devise a suction cup to overcome the above shortcoming.